## Warning: package 'leaflet' was built under R version 3.3.2
## [1] "SpatialPolygonsDataFrame"
## attr(,"package")
## [1] "sp"
##  [1] "GEOID10"    "STUSPS10"   "NAME10"     "ALAND10"    "AWATER10"  
##  [6] "INTPTLAT10" "INTPTLON10" "median_age" "Shape_Leng" "Shape_Area"